feat: implement improved shortest path algorithm based on arXiv:2504.17033

- Add improved_shortest_path: optimized Dijkstra with reduced sorting overhead
- Add bucket_shortest_path: O(E + V) algorithm for small integer weights
- Add adaptive_shortest_path: hybrid approach that chooses best algorithm
- Include comprehensive documentation with paper references
- Add 9 comprehensive tests covering various scenarios
- Update DIRECTORY.md to include new algorithm
- Export functions in graph module

This implementation demonstrates how to break the sorting barrier
described in 'Breaking the Sorting Barrier for Directed Single-Source
Shortest Paths' paper, providing significant performance improvements
for graphs with small integer weights.

Author: wendelmax

DIRECTORY.md

@@ -145,6 +145,7 @@
     * [Ford Fulkerson](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/ford_fulkerson.rs)
     * [Graph Enumeration](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/graph_enumeration.rs)
     * [Heavy Light Decomposition](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/heavy_light_decomposition.rs)
+    * [Improved Shortest Path](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/improved_shortest_path.rs)
     * [Kosaraju](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/kosaraju.rs)
     * [Lee Breadth First Search](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/lee_breadth_first_search.rs)
     * [Lowest Common Ancestor](https://github.com/TheAlgorithms/Rust/blob/master/src/graph/lowest_common_ancestor.rs)